Field of the Invention
The present invention relates to dye-sensitized solar cells and fabrication method thereof, and particular, to dye-sensitized solar cells using more than one layer of semiconductor nanofibers as photoanode.
Description of the Related Art
The worldwide demand for energy is expected to double by the year 2050 and triple by the end of the century. Abundant supply of clean energy is necessary for global political, economical and environmental stability. The development of carbon-free source of sustainable renewable energy is one of the major challenges for scientists this century, including wind power, atomic energy and solar energy. Photovoltaic cells utilize practically inexhaustible solar energy, and are environmentally friendly. Since the first selenium photovoltaic cell was developed in 1983, silicon based solar cells have drawn a great deal of interest, and the technologies have been efficiently developed. However, conventional solar cells are not economically available due to high fabrication cost. Moreover, there are some limitations in the practical application and improvement in the efficiency.
Dye-sensitized solar cell (DSSC) is a non-conventional photovoltaic technology that has attracted much attention due to its cost-effectiveness in harvesting solar energy with appealing properties such as flexibility, transparency, and adaptability in large-area devices. The operating principle of DSSC is illustrated in FIG. 1. Upon illumination, the dyes adsorbed onto the metal oxide semiconductor (usually TiO2) are sensitized to the excited state (S*) by light absorption right at the interface and they dissociate readily to create an electron-hole pair, with electrons subsequently injected into the conduction band of the semiconductor while the holes, at least initially, remain on the sensitizers. The dye ground state (S) is then regenerated by electron donation from the redox system to the oxidized state of the sensitizer (S+). The recuperation of redox system is realized by transporting holes to the counter electrode either in diffusion or hopping mechanism (depending on the transporting mediator). The whole process is finally completed by electron migration via the outer circuit and the device generates electric power from light without chemical transformation.
In early development, there seemed little scope for practical application of DSSC because the currents generated by sensitization of single crystal electrodes such as zinc oxide are very small because the dye is present only as a monolayer at the surface and light absorption is therefore very weak. Until in 1991 O'Regan and Gratzel published a remarkable report: 7% efficiency DSSC fabricated using a nanocrystalline titanium dioxide sensitized by a strongly absorbed ruthenium dye. Since then, efforts to optimize DSSC have resulted in cells with efficiencies above 11% (under extreme ideal situation), which has lasted for more than a decade. Considerable efforts have been performed with an attempt to further improve the performance of DSSC for successful commercialization. These include:                (1) increasing the light harvesting (nabs). There are numerous methods to increase the light harvesting, such as increase surface area of the semiconductor, develop new dyes and dyes mixture with strong and broad absorption spectrum, introduce photonic crystal or address a light-scattering layer on the top of the photoanode.        (2) improving the electron injection efficiency (ηinj) and collection efficiency (ηcoll). Fast diffusion and low recombination can improve injection and collection efficiency. Various methods have been carried out to transfer electron with the titanium in preference to other decay channels, for example modified surface of TiO2 with insulating oxides or high band gap semiconductors, post-treatment with aqueous TiCl4 solution, or use functional nanostructured photoanodes like one-dimensional nanostructures (nanotubes, nanowires, nanofibers).        
The most broadly researched DSSC photoanode is composed of a mesoporous metal oxide semiconductor (usually TiO2) fabricated form sol-gel processed sintered nanoparticles (20 nm in diameter) and addition of light scattering layer (200-400 nm in diameter) coated on transparent conductive oxide, e.g. Fluorine-doped SnO2 (FTO) glass. However, electron transport in nanoparticle based DSSC photoanode mainly rely on trap-limited diffusion process, a low mechanism that limits the efficiency of the device. One promising solution is to provide more direct pathway for electron transport by replacing the nanoparticle with one-dimensional nano-materials, such as nanofibers as photoanode. This would help to reduce the recombination of the electron-hole pair and at the same time improve the transport of electron to the conducting glass and thereafter to the external circuit. In conjunction, it is important to find an effective media to better reflect the light and use the trapped light for further energizing the dye in the energy-harvesting layer of the photoanode. In the past, the selection of the energy harvesting layer and the reflector layer are unrelated and often this also results in poor performance (in efficiency) and much higher fabrication costs as it is at least a 2-step process.
In view of the conventional dye-sensitized solar cells, there still exists a need for high efficiency solar cells that are both simple to make and cost effective.